[Tyneras]

My general hypothesis is this:

Similar (in some complicated non-intuitive way) timelines cluster. Excessively similar timelines merge, so all existing lines are different in some significant manner or are in danger of merging soon. Conversely, more divergent timelines are more distant and difficult to access. The current mix of accessible timelines means that the fundamental principles of timeline similarity are not understood well, even by Centrum.

Doppelgangers are the result of timelines being near each other and interfering with each other. Maybe Hitler didn't exist on Homeline until a Reich timeline got too close.

Where are the aliens?

The most common timeline involves earth being a sterile, lifeless ball. This is the same for all potentially inhabited planets. Cascading merging means that virtually all timelines have a single inhabited planet (which might be earth or might not) in an otherwise sterile universe.

This works because timeline merging can be partial and messy rather than total and perfect, which is where reality shards come from. Alien artifacts from aliens that never existed in this universe, but did visit a dead and sterile earth in their universe prior to merging and reality quakes, for example.